US secretary of state John Kerry said Nigeria's president Goodluck Jonathan was happy to have America's support immediately.

"Our embassy in Abuja is prepared to form a coordination cell that can provide expertise on intelligence, investigations and hostage negotiations and to help facilitate information sharing and victims' assistance," Mr Kerry said.

"We remain deeply concerned about the welfare of these young girls and we want to provide whatever assistance is possible to provide for their safe return to their families."

Mr Kerry was asked at press conference in Washington why it had taken more than three weeks for the US to respond.

"We have been in touch since day one and we have been engaged," Mr Kerry said.

"But the government (of Nigeria) had its own set of strategies, if you will, in the beginning. You can offer and talk, but can't do that if a government has its own sense of how it's proceeding.

"I think now the complications that have arisen have convinced everybody that there needs to be a greater effort. It will begin immediately, literally immediately."

"We've been talking to AFRICOM (US Africa Command) and I think you'll see a very rapid response."

Nigerian students protest in Washington DC

Nigerian students who are studying in the US and their supporters have marched on the Nigerian embassy in Washington.

They chanted "bring them back to safety" and "silence saves no-one".

Their signs read "our children deserve better..." and "bring our girls back now".

Rally organiser Lola Adele-Oso called for Nigeria's government to take action to save the girls and end sex trafficking.